vaapivideocontext: check if query context is NULL
vaapivideocontext: check if query context is NULL

Under certain conditions the element might receive a positive context query
but without a context instance. This situation will lead to a segmentation
fault when traversing the context list in the pipeline.

https://bugzilla.gnome.org/show_bug.cgi?id=767946

01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi